The present invention relates to an optical fiber functional device and a method of manufacturing the same, and more particularly relates to an optical fiber functional device capable of being used to form a parallel beam converter system in disposing a functional optical element such as an optical isolator in an optical fiber wave-guide for optical communication, and a method of manufacturing the same.
It is known that if a semiconductor laser unit, which is a main light source for optical fiber communications, optical disk input and output or the like, receives reflected light back from the end of an optical fiber connected to the laser unit, the point of connection of optical fibers, or an optical system such as a coupling lens and an optical connector, the oscillation of the unit becomes so unstable that it undergoes a large operational deterioration such as an increase in noise and a fluctuation in output. Therefore, various optical isolators have been developed to prevent the oscillation of the semiconductor laser unit from becoming unstable due to the reception of the reflected light, to make the unit be a stable light source for optical communication.
Although an optical isolator including a Rochon's prism as a polarizer or an analyzer, a Faraday-effect rotator made of a single crystal of YIG (yttrium-iron garnet), bismuth-substituted yttrium-iron garnet or the like, and a holed permanent magnet made of SmCo or the like to magnetize the rotator in the forward direction is widely known, the isolator is only effective for a certain plane of polarization, and has a problem that if light not coincident with the direction of polarization of the isolator enters into it, the light undergoes a large loss in passing through the isolator. For use between optical fibers, an optical isolator with no dependence on polarization has been desired because a light beam, which is transmitted through the optical fibers, has generally the changed plane of polarization.
Thus, various optical isolators, in each of which the separation and/or synthesis of ordinary and extraordinary rays obtained by a flat plate of double refraction crystal such as calcite or an artificial anisotropic medium instead of a Rochon's prism are utilized to eliminate almost all of the loss of light in the forward direction as to all the planes of polarization so as to make the isolator not dependent on polarization, have been proposed.
FIG. 1 shows a conventional optical isolator disclosed in Japanese Patent Publication No. 28561/83 and including a lens 10, two flat plates 11 and 12 of double-refraction rutile crystals, a Faraday-effect rotator 13 which is a magnetooptic member, and an optically rotatory plate 14 of optically rotatory or anisotropic crystal such as quartz.
In the isolator, the direction of polarization of light from an optical fiber 8 as a light passage is changed by an angle of 45 degrees clockwise by each of the rotator 13 and the optically rotatory plate 14. The flat plates 11 and 12 of double-refraction rutile crystals have the same thickness, and are tilted by a prescribed angle to the optical axis of the isolator so that the axes of the plates are parallel with each other to prevent light from proceeding from another optical fiber 9 to the optical fiber 8.
FIG. 2 shows another conventional optical isolator disclosed in the Japanese Patent Publication No. 58809/86 and including lenses (a) and (b), two double-refraction crystal plates 11, and a Faraday-effect rotator 13. Each of the plates 11 is shaped as a wedge. The oblique sides of the plates 11 face each other across the Faraday-effect rotator 13. Shown at 1 and 4 in FIG. 6 are optical fibers as light passages. The isolator functions nearly in the same manner as that shown in FIG. 1.
Since the diameter of each of the lenses provided in the above-mentioned conventional optical isolators in order to cause each of them to function as a fiber collimator to transmit the light from one of the optical fibers to the other is much larger than that of the fiber, the entire size of the isolator is large. This is a problem. Since the distance from the light outlet end of one of the optical fibers to the nearby lens of the optical isolator and that from the other lens thereof to the light inlet end of the other of the fibers need to be optimized while the state of the optical path for the light transmitted through the lens and the intensity of the light are monitored, it takes much time and trouble to assemble the fiber collimator. This is also problematic.
FIG. 3 shows a conventional optical fiber collimator in which spherical lenses 23 and 24 are connected to the mutually opposed ends of optical fibers 21 and 22 so that the rays of light transmitted through one of the fibers are made parallel with each other. FIG. 4 shows another conventional optical fiber collimator in which rod lenses 25 and 26 of the refractive index distribution type are connected to the mutually opposed ends of optical fibers 21 and 22 so that the rays of light transmitted through one of the fibers are made parallel with each other.
However, from standpoints of the efficiency and cost of production, the conventional optical fiber collimators have problems that the locations of the optical fibers and the lenses need to be modulated and fixed with the accuracy of the micron order, and anti-reflection coatings need to be provided on the lenses to prevent the light from being reflected due to the difference between the refractive indices of each components.